To raise awareness of the possibility of spontaneous temporal bone cerebrospinal fistula in case of clear retrotympanic effusion.

A 63-year-old man with no particular history presented with unilateral spontaneous right retrotympanic clear effusion. CT found defects in the posterior part of the right temporal bone, in contact with arachnoid granulations, with no other visible abnormalities.

Unilateral clear retrotympanic effusion in an adult subject should, apart from serous otitis media, suggest possible cerebrospinal fistula. In the absence of otologic or traumatic history, arachnoid granulation is one possible etiology, inducing spontaneous cerebrospinal fluid leakage when facing the temporal bone. Diagnosis is suggested by bone defects in the tegmen tympani or posterior wall of the temporal bone on CT, with the adjacent mastoid cavities filled with fluid. Pneumococcal vaccination and early surgical repair of the fistula should be performed to avoid neuromeningeal infection.